In rolling mills for grinding bulk materials it is known to adjust the grinding gap. This grinding gap determines, among other things, the maximum particle size of the bulk material that is passed between two rolls. It is to be understood, however, that below this maximum size the most diverse particle sizes will be possible, even though in general a relatively narrow particle size spectrum is aimed at. Such a control of a roll mill is described in U.S. Pat. No. 3,468,488, by way of example.
From U.S. Pat. No. 3,716,196 it is also known to regulate a hammer mill in such a way that the product is thereafter subjected to a simple screening with a subsequent determination of the share of the oversizes. Further, in this case a maximum particle size is also given by the mesh size of the sieve, whereas the smaller particle sizes will not be taken into consideration for the control.
Finally, CH-PS 438 760 may be mentioned from which it is known to measure the size of gaps (but not of solid bodies) in a pneumatic manner.
Even if one came to realize that also the determination of the smaller particle sizes is important for a lot of processes, this would generally be determined by way of multiple sievings, which is a time-consuming process.